With a tomosynthesis method a conventional x-ray tube and a flat x-ray detector are used. The x-ray tube is moved successively into a plurality of angular positions. An angle can be defined as the angle between a connecting line from the x-ray tube to a (selected, typically central) point on the flat x-ray detector and the surface normal of the flat x-ray detector (which would not be a flat x-ray detector without the definition of a surface with a surface with a surface normal). At least one x-ray image is acquired in each angular position by switching the flat x-ray detector to sensitive over the period of a time window and emitting x-ray radiation by way of the x-ray tube within the time window. With a tomosynthesis method the additional features must be present that the individual images are each acquired with a different focus of the x-ray radiation and then combined to form an overall image. These features are however of no further relevance in the context of the method claimed here.
The most important thing here is that different x-ray images are acquired based on different angular positions of the x-ray tube.
Depending on the angle setting, the x-ray radiation travels through different distances at the patient and at the patient bed. The absorption in the patient and patient table therefore varies with the angular position. Therefore the power arriving at the flat x-ray detector is variable. It can happen that the flat x-ray detector is not operated in an optimum mode, for example because the image signals are too weak. Flat x-ray detectors have different working areas, which can be roughly divided into a working area, wherein the received dose is too low, with the result that non-linear effects occur, a working area, wherein the received dose is a mean dose, with the result that the required linearity of the received signals is achieved with the received x-ray dose, and a working area, wherein the x-ray dose is too high, with the result that non-linear effects occur here too.
It is desirable for the flat x-ray detector to operate essentially in the middle working area. It might be an obvious solution to vary the power of the x-ray tube, to obtain correspondingly variable x-ray doses at the flat x-ray detector. However this would mean that the individual images acquired in the context of the tomosynthesis would no longer be comparable. As mentioned above, these are to be combined to form an overall image.